1. Field of the Invention
The present invention relates to implants made of metallic materials for use in the human or animal body.
2. Description of Related Art
Implants of this type have in principle been known for a long time. The first implants were developed for orthopaedic purposes, for example screws and nails for fixing bone fractures. These initially consisted of relatively simple iron alloys which were prone to corrosion under in vivo conditions. The corrosion led to metals being released as ions in the direct vicinity of the bone providing an unwanted stimulus for the growth of bone tissue. The bone grew more than is actually wanted and necessary. This led to damage to healthy bone material.
For this reason, attempts have been made to fabricate metallic implants in principle from materials with maximum corrosion resistance. Currently in use in this connection are mainly corrosion-resistant stainless steels, tantalum and titanium. These implants persist as foreign bodies after the implantation and they are recognized as such by the body. They can be removed only by a second operation.
In addition, metallic implants are known in the specialty of vascular surgery and cardiology, angiology and radiology. These implants comprise, for example, endoluminal and vessel supports (stents) for treating lesions. These supports are used, for example, for widening and maintaining the lumen of narrowed vessels by keeping the vessel lumen at an appropriately optimal internal diameter using a balloon catheter (balloon expandable) or self-expanding from the vessel lumen outwards. The implant is intrinsically necessary only until the diseased vessel is able permanently to maintain the necessary diameter under its own power due to biological repair processes. This is generally the case about 4 weeks after implantation.
However, some disadvantages are associated with permanent retention of a metallic implant. As a foreign body, the implant leads to local and possibly also systemic reactions. In addition, the self-regulation of the affected vessel segment is impeded. The continual (pulsatile) stress on the metal may lead to fatigue fractures, which in the case of large-lumen implants (e.g. occlusion systems such as umbrellas) may lead to new medical problems. About 20% of vessel supports in smaller lumina (2.5-6 mm) cause renewed stenosis (called in-stent stenosis) which, given the large number of implants, accumulates to an additional medical and economic burden. In some vessel regions (e.g. extracranial vessels, leg arteries) the metallic structure may be permanently deformed by forces acting from outside, resulting in renewed vessel obstruction or induced vessel occlusion. Every permanent implant is additionally associated with problems in particular for younger patients because retention for decades is unavoidable.
The only completely biodegradable implants disclosed to date, for example in DE 2502884 C2, are made of synthetic materials. There is disclosure therein of coating an orthopaedic implant with polymethylmethacrylate which is biodegradable. Other synthetic materials comprise polylactide and polyglycolic esters. In addition, EP 0006544 B1 discloses a biodegradable ceramic material based on calcium phosphate, which is likewise used for coating metallic implants.
Finally, WO 81/02668 discloses an orthopaedic implant which comprises a corrosion-resistant metallic basic component and a biodegradable intermediate metallic layer for the bone contact region. This intermediate layer forms together with the basic component an electrochemical cell and generates an electrical voltage which promotes bone growth. At the same time, the surface layer which may, for example, consist of silver alloys is degraded. This leads to the desired effect, that bone growth is beneficially influenced for as long as necessary and then, after complete degradation of the surface coating, the electrical stimulus declines.
Previously disclosed biodegradable polymer-based substances are used in vascular surgery. Their mechanical properties on the one hand and the subsequent foreign-body reaction during the biodegradation on the other hand lead to them being unsuitable on their own as material for implantation. Metallic materials/alloys have favourable mechanical properties (elasticity, deformability, stability) while being less bulky, which is an important precondition for administration through thin-lumen guide systems in the transcutaneous procedure.